As is well known, nitriding or carbonitriding processes in the gaseous phase are used to improve the fatigue and siezure characteristics and to increase the resistance to wear and corrosion of steels.
These processes involve maintaining the steel parts in an atmosphere containing 50% to 100% of gaseous ammonia at a temperature of 490.degree. C. to 750.degree. C. Parts treated in this way are then cooled by quenching in a gaseous or liquid phase. At the process temperatures used, the gaseous ammonia decomposes partially in contact with the steel parts, and the nitrogen thus released becomes a solid-phase solution and diffuses through a certain depth of the steel. It thus forms a nitrided layer on the surface of the part, consisting of a so-called "combination layer" or "white layer" on a substrate consisting of a so-called "diffusion layer". The structure and composition of these layers is as follows:
The white layer, situated at the extreme surface of the part, consists of of a mixture of .xi. nitrides (Fe.sub.2 N--Fe.sub.3 N) and .gamma.' nitrides (Fe.sub.4 N). PA0 The diffusion layer, which lies under the white layer, is less hard and more thick than the latter; in this diffusion layer, the nitrogen is in a solid insertion solution and forms nitrides by combining with certain elements existing in the steel. PA0 The white layer develops non-uniformly and thus is of unequal thickness over the surface of the part. PA0 The white layer is fragile and does not adhere well because it consists of an intimate mixture of two compounds .xi. (Fe.sub.2 N--Fe.sub.3 N) and .gamma.' (Fe.sub.4 N). PA0 The white layer is present in the form of a compact zone covered with a porous zone, which can in some cases have a number of disadvantages. If the process time exceeds a certain period, the compact zone reaches a maximum limiting thickness (20 .mu.m) and the porous zone alone develops. PA0 The incubation time necessary for nucleation of the nitrides is fairly long and it thus requires a certain time for the white layer to form. PA0 The white layer formed has a constant thickness over the whole surface of the part to be treated. PA0 The white layer consists in general only of the .xi. phase (Fe.sub.2 N--Fe.sub.3 N), and is thus stronger. If, on occasion, for certain process durations and ammonia contents of the atmosphere used, the .gamma.' phase (Fe.sub.4 N) appears, the latter does not mix with the .xi. phase, but is present in the form of a thin zone sandwiched between the .xi. zone and the diffusion layer; under these circumstances, the presence of the .gamma.' phase does not affect the strength of the nitrided layer. PA0 The porous zone of the white layer is extremely thin and may even be non-existent in the case of short-duration treatments. PA0 The kinetics of the reaction are increased compared with conventional treatments, the incubation time necessary for nitride nucleation being shorter; if required, this enables the initial concentration of the gaseous ammonia mixture to be reduced to 10%, and the duration of the treatment to be shortened, and also enables the flow and renewal rate of the atmosphere in the process furnace to be reduced.
Nitriding atmospheres used at the present time consist either of gaseous ammonia, or of gaseous ammonia diluted with nitrogen, or of gaseous ammonia, where appropriate diluted with nitrogen, mixed with a compound which can increase the degree of dissociation of the ammonia on contact with the part, and thus the kinetics of nitriding; this compound may, for instance, be an oxidizing compound such as oxygen, carbon dioxide, steam, or a hydrocarbon. In certain processes, a mixture of gaseous ammonia and a carbon-bearing gas is used, for instance, an endothermic gas (CO--H.sub.2 --CO.sub.2 --H.sub.2 O--CH.sub.4 --N.sub.2 --) or a hydrocarbon; carbonitriding then occurs with the formation of .xi. carbonitrides. In all cases of processes used up to the present time, it was preferable to have an initial ammonia concentration in the atmosphere used of at least 50%.
For nitriding processes using ammonia mixed with an oxidizing compound utilised up to the present time, mention may be made specifically of the process described in British Pat. No. 2.049.740. This process consists of holding steel parts at a temperature of 550.degree. C. to 600.degree. C. in a gaseous atmosphere consisting of at least 50% by volume of ammonia, carbon dioxide and nitrogen (e.g. 70% NH.sub.3, 7% CO.sub.2 and 23% N.sub.2), for a duration of 12 to 20 hours.
All these processes enable surface hardening of the parts treated to be obtained, but do however have a number of drawbacks relating to the white layer. This is because: